Liquid crystals have been used in a variety of display devices, particularly that type of liquid crystal known as nematic. This type of liquid crystal when in the mesomorphic phase is normally transparent. However, when a voltage is impressed across the liquid crystal material, where the liquid crystal is of the type that has a dipole moment which is transverse to the main axis of the liquid crystal molecule, then the liquid crystal material becomes light-dispersive, as a result of which it loses its transparency. By proper selection of portions of a display panel including liquid crystal materials of this type, it then becomes possible to make a display which becomes visible as a result of dispersion of ambient light.
Such displays using the more common type of nematic crystal are equally visible from all directions. Moreover, in general, they have optical activity so that the direction of plane-polarized light can be rotated. As is usual, the degree of rotation produced by a display device containing an optically active compound depends on the specific rotation of the compound itself and the quantity of compound traversed by the light beam in question.
Given a specific device, the rotation of the device will be fixed, depending on the construction, i.e., the specific rotation of the compound used and the concentration of the material and the length of the light path through the device. There is no way of affecting or varying the degree of rotation with such a device. However, it would be desirable to be able to do so.